Location and temporal memory of objects declines in aged marmosets (Callithrix jacchus)

Atrophic astrocytes in aged marmosets present tau hyperphosphorylation, RNA oxidation, and DNA fragmentation

Astrocytes perform multiple essential functions in the brain showing morphological changes. Hypertrophic astrocytes are commonly observed in cognitively healthy aged animals, implying a functional defense mechanism without losing neuronal support. In neurodegenerative diseases, astrocytes show morphological alterations, such as decreased process length and reduced number of branch points, known as astroglial atrophy, with detrimental effects on neuronal cells. The common marmoset (Callithrix jacchus) is a non-human primate that, with age, develops several features that resemble neurodegeneration. In this study, we characterize the morphological alterations in astrocytes of adolescent (mean 1.75 y), adult (mean 5.33 y), old (mean 11.25 y), and aged (mean 16.83 y) male marmosets. We observed a significantly reduced arborization in astrocytes of aged marmosets compared to younger animals in the hippocampus and entorhinal cortex. These astrocytes also show oxidative damage to RNA and increased nuclear plaques in the cortex and tau hyperphosphorylation (AT100). Astrocytes lacking S100A10 protein show a more severe atrophy and DNA fragmentation. Our results demonstrate the presence of atrophic astrocytes in the brains of aged marmosets.

The common marmoset as a model of neurodegeneration

Human life expectancy has increased over the past few centuries, and the incidence of dementia in the older population is also projected to continue to rise. Neurodegenerative diseases are complex multifactorial conditions for which no effective treatments are currently available. Animal models are necessary to understand the causes and progression of neurodegeneration. Nonhuman primates (NHPs) offer significant advantages for the study of neurodegenerative disease. Among them, the common marmoset, Callithrix jacchus, stands out due to its easy handling, complex brain architecture, and occurrence of spontaneous beta-amyloid (Aβ) and phosphorylated tau aggregates with aging. Furthermore, marmosets present physiological adaptations and metabolic alterations associated with the increased risk of dementia in humans. In this review, we discuss the current literature on the use of marmosets as a model of aging and neurodegeneration. We highlight aspects of marmoset physiology associated with aging, such as metabolic alterations, which may help understand their vulnerability to developing a neurodegenerative phenotype that goes beyond normal aging.

Spontaneous object recognition in capuchin monkeys: assessing the effects of sex, familiarization phase and retention delay

The spontaneous object recognition (SOR) task is a versatile and widely used memory test that was only recently established in nonhuman primates (marmosets). Here, we extended these initial findings by assessing the performance of adult capuchin monkeys on the SOR task and three potentially intervening task parameters-object familiarization phase, retention delay and sex. In Experiment 1, after an initial 10-min familiarization period with two identical objects and a pre-established retention delay (0.5, 6 or 24 h), the capuchins preferentially explored a new rather than the familiar object during a 10-min test trial, regardless of delay length. In Experiment 2, the capuchins were again exposed to two identical objects (but now for 10 or 20 min), then a 30-min retention delay and a 10-min test trial. An exploratory preference for the new over the familiar item was not affected by the length of the familiarization interval, possibly because overall exploration remained the same. However, the amount of initial object exploration was not related to task performance, and both males and females performed similarly on the SOR task with a 10-min familiarization, 30-min delay and 10-min test trial. Therefore, male and female capuchins recognize objects on the SOR task after both short and long delays, whereas a twofold increase in the familiarization phase does not affect task performance. The results also provide further support for the use of incidental learning paradigms to assess recognition memory in nonhuman primates.

Probing the nature of episodic memory in rodents

Episodic memory (EM) specifies the experience of retrieving information of an event at the place and time of occurrence. Whether non-human animals are capable of EM remains debated, whereas evidence suggests that they have a memory system akin to EM. We here trace the development of various behavioral paradigms designed to study EM in non-human animals, in particular the rat. We provide an in-depth description of the available behavioral tests which combine three spontaneous object exploration paradigms, namely novel object preference (for measuring memory for "what"), novel location preference (for measuring memory for "where") and temporal order memory (memory for "when"), into a single trial to gauge a memory akin to EM. Most important, we describe a variation of such a test in which each memory component interacts with the others, demonstrating an integration of diverse mnemonic information. We discuss why a behavioral model of EM must be able to assess the ability to integrate "what", "where" and "when" information into a single experience. We attempt an interpretation of the various tests and review the studies that have applied them in areas such as pharmacology, neuroanatomy, circuit analysis, and sleep. Finally, we anticipate future directions in the search for neural mechanisms of EM in the rat and outline model experiments and methodologies in this pursuit.

Age-Related Learning and Working Memory Impairment in the Common Marmoset

Aging is the greatest risk factor for the development of neurodegenerative diseases, yet we still do not understand how the aging process leads to pathologic vulnerability. The research community has relied heavily on mouse models, but the considerable anatomic, physiological, and cognitive differences between mice and humans limit their translational relevance. Ultimately, these barriers necessitate the development of novel aging models. As a nonhuman primate (NHP), the common marmoset (Callithrix jacchus) shares many features in common with humans and yet has a significantly shorter lifespan (10 years) than other primates, making it ideally suited to longitudinal studies of aging. Our objective was to evaluate the marmoset as a model of age-related cognitive impairment. To do this, we used the Delayed Recognition Span Task (DRST) to characterize age-related changes in working memory capacity in a cohort of sixteen marmosets, of both sexes, varying in age from young adult to geriatric. These monkeys performed thousands of trials over periods of time ranging up to 50% of their adult lifespan. To our knowledge, this represents the most thorough cognitive profiling of any marmoset aging study conducted to date. By analyzing individual learning curves, we found that aged animals exhibited delayed onset of learning, slowed learning rate after onset, and decreased asymptotic working memory performance. These findings are not accounted for by age-related impairments in motor speed and motivation. This work firmly establishes the marmoset as a model of age-related cognitive impairment.SIGNIFICANCE STATEMENT Understanding the normal aging process is fundamental to identifying therapeutics for neurodegenerative diseases for which aging is the biggest risk factor. Historically, the aging field has relied on animal models that differ markedly from humans, constraining translatability. Here, we firmly establish a short-lived nonhuman primate (NHP), the common marmoset, as a key model of age-related cognitive impairment. We demonstrate, through continuous testing over a substantial portion of the adult marmoset lifespan, that aging is associated with both impaired learning and working memory capacity, unaccounted for by age-related changes in motor speed and motivation. Characterizing individual cognitive aging trajectories reveals inherent heterogeneity, which could lead to earlier identification of the onset of impairment, and extended timelines during which therapeutics are effective.

A single brief stressful event time-dependently affects object recognition memory and promotes familiarity preference in marmoset monkeys

A stressful experience can enhance information storage and impair memory retrieval in the rodent novel object recognition (NOR) task. However, recent conflicting results underscore the need for further investigation. Nonhuman primates may provide a unique, underexplored and more translational means to investigate stress-mediated changes in memory. Therefore, we assessed whether a single brief extrinsic stress event affects information encoding, storage and/or retrieval in adult marmoset monkeys submitted to the NOR task. This consisted of an initial 10 min familiarization period with two identical neutral objects. After a 6 h delay, a 10 min test trial was held where a new and familiar object could be explored. Stress was induced by a 15 min restraint event held before or after the encoding phase, or prior to retrieval. Pre-encoding stress had no effect on task performance, as this group displayed above-chance novelty preference similar to non-stressed controls. Post-encoding stress induced memory deficits, with both objects being explored equally. Interestingly, pre-retrieval stress induced an above-chance familiarity preference. A single brief stressful event thus affects recognition memory in a time-dependent manner. Also, negative discrimination ratios can be used as a measure of memory in the NOR task and a change in strategy may not mean memory failure in spontaneous learning paradigms.

Anatomical variability, multi-modal coordinate systems, and precision targeting in the marmoset brain

Localising accurate brain regions needs careful evaluation in each experimental species due to their individual variability. However, the function and connectivity of brain areas is commonly studied using a single-subject cranial landmark-based stereotactic atlas in animal neuroscience. Here, we address this issue in a small primate, the common marmoset, which is increasingly widely used in systems neuroscience. We developed a non-invasive multi-modal neuroimaging-based targeting pipeline, which accounts for intersubject anatomical variability in cranial and cortical landmarks in marmosets. This methodology allowed creation of multi-modal templates (MarmosetRIKEN20) including head CT and brain MR images, embedded in coordinate systems of anterior and posterior commissures (AC-PC) and CIFTI grayordinates. We found that the horizontal plane of the stereotactic coordinate was significantly rotated in pitch relative to the AC-PC coordinate system (10 degrees, frontal downwards), and had a significant bias and uncertainty due to positioning procedures. We also found that many common cranial and brain landmarks (e.g., bregma, intraparietal sulcus) vary in location across subjects and are substantial relative to average marmoset cortical area dimensions. Combining the neuroimaging-based targeting pipeline with robot-guided surgery enabled proof-of-concept targeting of deep brain structures with an accuracy of 0.2 mm. Altogether, our findings demonstrate substantial intersubject variability in marmoset brain and cranial landmarks, implying that subject-specific neuroimaging-based localization is needed for precision targeting in marmosets. The population-based templates and atlases in grayordinates, created for the first time in marmoset monkeys, should help bridging between macroscale and microscale analyses.

Invasive Research on Non-Human Primates-Time to Turn the Page

Simple Summary

Despite increasing ethical concerns, primates are still often used in invasive research (i.e., laboratory research that causes body manipulations causing them pain or distress and not aimed at directly improving their well-being). Here, we will review previous studies showing that primates have complex behaviour and cognition, and that they suffer long-term consequences after being used in invasive research. We will discuss the ethical problems that invasive research on primates posit, the legal protection that they are, to date, granted in different countries, and summarize the past and current attempts to ban this kind of research on primates. We will conclude why, in our opinion, invasive research on primates should be banned, and non-invasive methods should be considered the only possible approach to the study of primates.

Abstract

Invasive research on primates (i.e., laboratory research that implies body manipulations causing pain or distress that is not aimed to directly improve the individuals’ well-being) has a long history. Although some invasive studies have allowed answering research questions that we could not have addressed with other methods (or at least not as quickly), the use of primates in invasive research also raises ethical concerns. In this review, we will discuss (i) recent advances in the study of primates that show evidence of complex behaviour and cognition, (ii) welfare issues that might arise when using primates in invasive research, (iii) the main ethical issues that have been raised about invasive research on primates, (iv) the legal protection that primates are granted in several countries, with a special focus on the principle of the 3Rs, and (v) previous and current attempts to ban the use of primates in invasive research. Based on this analysis, we suggest that the importance of a research question cannot justify the costs of invasive research on primates, and that non-invasive methods should be considered the only possible approach in the study of primates.